The present invention relates to the stabilization of a loop of web having notches, holes or other openings along the web as it passes through a vacuum box, and more specifically, to the provision of a flexible ring that is positioned in the loop of web material.
Vacuum boxes have long been used for applying tension to a loop of a web material, such as webs of paper or photographic film. One application of a vacuum box is along the path between a supply of web material and a station where the web material is wound onto a spool. A loop of the web material is formed in the vacuum box by applying a vacuum to the bottom of the box, and the vacuum holds the loop in the box under slight tension. The web material can be driven out the box intermttently during the spooling operation, and can be fed into the box from the web supply continuously or on an intermittent basis. It is known to provided sensors to detect the position of the bottom of the loop in the vacuum box and the shut off the drive mechanism between the supply and the vacuum box when the loop reaches a predetermined maximum size.
If the web passing through the vacuum box has one or more notches or other openings in it, the web will vibrate excessively as it passes through the box, and there will be a dramatic loss in tension in the web induced by the vacuum box when the notch passes through the bottom of the loop in the vacuum box. The vibration and the loss in tension result from a surge of air flowing through the notch. Such vibrations and the change in web tension are most undesirable and can adversely effect operations downstream of the vacuum box.
U.S. Pat. No. 3,871,597, issued Mar. 18, 1975 in the name of H. LaMers, relates to a labeling apparatus that uses a web carrying a plurality of labels. A loop of the web is formed in a chamber, and vacuum is applied to the chamber to draw the web into a supply loop in the chamber. A curtain in the chamber maintains the web in the loop configuration. The curtain is described as comprising an thin flexible material, such as a 0.005 inch thick sheet of nylon, which is drawn by the vacuum and presses against the web to maintain it as a loop. One end of the curtain is fixed to a post while the opposite end is free and in constant engagement with the web. The web is prepared with die cuts on the border of each label area to be formed to facilitate separation of the labels from the rest of the web. Each label area is defined by a plurality of cuts which forms a closed loop that encircles the label area. The areas at the edges of the web and outside the label areas have sprocket holes that are spaced along the length of the web. The curtain is apparently imperforate and is described as sealing the web area where cuts are made that form the label and sprocket areas. The curtain is designed to fill out the full width of the supply chamber when the web is narrower than the chamber, and in addition, it is an opaque material which can be used to interrupt a light beam used for detecting and controlling the size of the loop in the chamber.
U.S. Pat. No. 3,807,612 issued Apr. 30, 1974 in the name of R. H. Eggert relates to a web feeding apparatus for a blank making machine. The Eggert patent discloses a vacuum box in which the size of the loop of web material in the box is sensed by a series of lamps and related photo cells. As stated in column 10 of the patent, when the web material is transparent the loop control system may be inefficient, inprecise or ineffective. In order to avoid this problem an opaque shade of thin flexible material is suspended inside the loop of web material. One end of the shade is secured to a rod and the other end is free to move into engagement with the web loop. The shade need only be wide enough to eclispe the photocells from the lamps and hence can have a maximum width that is only somewhat less than the narrowest strip of the web material that is to be drawn into the vacuum box. The length of the shade is adequate to reach the bottom of the loop when the loop is enlarged enough the receive light from the lowermost photocell of the system. The differential air pressure across the loop causes the shade to substantially conform to the interior surface of the loop.
The curtain or shade in the patented devices discussed above are both fixed to a rod or the like at one end and, as a result, there is relative motion between the web and the curtain or shade. This relative motion can effect scratching of the web. In addition, the fact that one end of the curtain or shade is secured to a rod may prevent the use of such devices in vacuum boxes where the size of the loop of the web in the box varies significantly during rapid intermittent advancement of a long strip of web out of the box, as in a film or paper winding operation. Also, the curtain or shade extends not only to the loop area of the web, but also through the area where the web enters and leaves the vacuum box. Such can complicate threading of the web into the vacuum box. In addition, as the curtain or shade of the prior devices is moved up or down with the loop in the web, the tension exerted by the curtain or shade appears to increase and decrease, thus resulting in a constantly varying tension on the web. This conflicts with an important result desired for a vacuum box, i.e. that it be effective to maintain a substantially constant tension on the web in the box.